Very large, multiple-beam satellite antennas in the 20 to 55 m range in size have been planned for mobile communication outlined in a paper presented at the 35th Annual International Astronautics Federation Congress, Laussane, Switzerland, Oct. 10, 1984, titled "NASA's Mobile Satellite Communications Program; Ground and Space Segment Technologies." To cover the continental United States, from 40 to 90 contiguous beams are to be generated from overlapping cluster feed arrays with diameters of up to six meters. In order to be compatible with the launching vehicle for the satellite, the antenna arrays should have the capability of being folded and stowed in the launch vehicle. Consequently, the array should be low in profile and light in weight. The problem is then to provide an array of elements for a satellite antenna with sufficient broadband performance to meet the requirements of the land mobile communications system and at the same time meet the low profile and lightweight requirements of the satellite.
A paper, titled "A Technique for an Array to Generate Circular Polarization with Linearly Polarized Elements" by the present inventor published in IEEE Transactions on Antenna and Propagation, Vol. AP-34, No. 9, Sep. 1986, pp. 1113-1124, presents theoretical and experimental results that demonstrate an array which generates circular polarization with wide axial ratio bandwidth (10 percent) can be constructed with single feed linearly polarized elements. The following paragraphs are essentially excerpted from that paper.
The reason that a circularly polarized array can be constructed by linearly polarized elements is primarily attributed to a four-element subarray with unique angular and phase arrangements. This basic subarray has its elements arranged in a 2.times.2 square or rectangular grid configuration with element angular orientation and feed phase arranged in either a 0.degree., 90.degree., 0.degree., 90.degree. or a 0.degree., 90.degree., 180.degree., 270.degree. relationship. The purpose of different angular orientations of the patches is to generate two orthogonally polarized fields, while different feed phases are used to provide the required phase delays for the desired circular polarization. It is well known that circular polarization can be achieved in the broadside direction of an array composed of two linearly polarized elements with angle and phase arranged in a 0.degree., 90.degree. relationship as shown in FIG. 1b for the bottom two patches. The same relationship is used for the other two elements, but reversed.
When only two linearly polarized (rectangular) patch elements are used and they are oriented at 90.degree. with respect to each other, the circular polarization becomes very poor at angles greater than 5.degree. off the broadside direction. This is caused by the spatial phase delay formed between the two orthogonally polarized elements. This spatial phase delay, which disturbs the required 90.degree. phase differential, contributes to the poor circular polarization quality at angles off broadside. With the 2.times.2 subarray, shown in FIG. 1a or 1b, this spatial phase delay no longer exists. This is because, within the two principle planes, the spatial phase delay in one row or column is opposite to that of the other row or column and, consequently, they cancel each other.
With such a system, not only is the feed complexity reduced, but also the bandwidth performance is improved. The reason for reduced feed complexity is because this technique only requires a single feed for each element while four feeds might be needed for each element in a conventional array with wide axial ratio bandwidth requirement. Due to the orthogonal orientation of neighboring elements, the mutual coupling effect is found to be significantly reduced from that of a conventional array. Finally, this uniquely arranged array can scan its main beam in the principle planes from its broadside direction to relatively wide angles without serious degradation of its circular polarization quality.
The concept presented in that paper is good for many different types of antenna elements, such as microstrip patches, dipoles, open-ended waveguides, horns, etc. However, this invention places special emphasis on the microstrip element as a result of a need for a large antenna array with a low profile and light weight. The present invention uses microstrip patches and the phase relationship between elements selected are as shown in FIG. 1b.
For a microstrip array antenna with a relatively thick substrate, there is a distinct advantage if the array antenna has its 2.times.2 subarrays arranged in the 0.degree., 90.degree., 180.degree., 270.degree. fashion for both its element orientations and feed phases as shown in FIG. 1b. In this fashion, the axial ratio bandwidth of the array can be increased substantially. This is because most of the radiation impurity (due to higher order modes of the thick substrate) from the 0.degree. element cancels that from the 180.degree. element and likewise for the 90.degree. and 270.degree. elements.
One important advantage found to be associated with the 2.times.2 array discussed here is that the mutual coupling of the array is significantly less than that of a conventionally arranged array. This is due to the fact that all the adjacent elements of this uniquely arranged array are orthogonally oriented and hence cause very little coupling between immediate neighboring elements.
The monolithic array with microstrip elements is playing an important role in the advance of phased array technology. It is more feasible to build a linearly rather than circularly polarized element with a quarter-wave long microstrip patch, and then provide a feed line circuit with phase shifters as required to realize 0.degree., 9.degree., 180.degree., 270.degree. phase relationship for circular polarization from a single feed point to the 2.times.2 subarray. With such a uniquely arranged subarray, it is possible to construct a circularly polarized monolithic phased array with enough room left for active devices to be etched on the same substrate.
The following describes the development of a new array which has a relatively broadband performance sufficient to cover the bandwidth requirements of the satellite antenna of the land mobile communication system. To cover both downlink frequencies (1545 to 1550 MHz) and uplink frequencies 1646 MHz to 1660 MHz) with a maximum of 1.5:1 input voltage standing wave ratio (VSWR), the microstrip antenna patch with a half-inch thick honeycomb-supported substrate has been selected as an example for the linearly polarized radiating element in arrays for circularly polarized radiation. Since a half-inch (0.07 wavelength) substrate is relatively thick for microstrip radiators. four feed probes would be required per single element of an array to suppress the undesired higher order modes and thus to generate acceptable circular polarization across the total bandwidth. For a large array, such a four-probe feed system for each microstrip patch would increase the complexity of an already complex feeding and beam-forming network, and would make it heavier and more prone to RF losses. For that reason, a 2.times.2 microstrip patch array is used and fed as a subarray from a single coaxial connector to function as a circularly polarized antenna element.